This invention relates, in general, to amplifiers, and more particularly to common base amplifiers.
A common base amplifier configuration is well known by those skilled in the art and has properties which make it very attractive for use as a buffer. The key properties are wide bandwidth and low input impedance. The low input impedance of the common base configuration allows it to be coupled to a large parasitic capacitance without forming a bandwidth limiting pole. Furthermore, the wide bandwidth of the common base amplifier allows it to be used in conjunction with other amplification stages with minimal impact to the subsequent amplifier stages. Generally, a common base stage provides a unity gain current signal from input to output thereby isolating a circuit coupled to the output from any input loading capacitance. Two problems which limit common base applications on an integrated circuit are 1) circuit complexity and 2) circuit density.
An example of a highly capacitive node is an input/output (I/O) of an integrated circuit. The high capacitance of an I/O can be the single most important factor in determining system speed of an integrated circuit. For example, optical signals have extremely fast transmission speeds. An optical signal coupled to an integrated circuit is converted to an electrical signal. The optical signal converted to an electrical signal typically has a small magnitude and must be amplified to logic levels used internally on the integrated circuit. Parasitic capacitance at the I/O form a dominant pole when coupled to an amplifier with a relatively high input impedance thereby limiting speed at which data can be transmitted to the input. This problem is compounded by the fact that hundreds of I/O's might be needed on the integrated circuit. I/O circuits are placed in close proximity to the I/O pad. Area for an I/O circuit is at a premium and any solution that addresses the speed problem must also be area efficient (to fit within an I/O site).
It would be of great benefit if a common base stage is provided which is extremely simple, occupies a small physical area, and has a wide bandwidth.